This invention relates to a continuous-wave radar system having a transmitter section, a receiver section, and a transmit/receive aerial which is common to both of said sections, said receiver section including a mixer arrangement for mixing a signal derived from said aerial and applied to a first input thereof with a signal derived from said transmitter section and applied to a second input thereof, the system including a directional coupler which couples an output of said transmitter section to said aerial via a signal path within said coupler from a first port to a second port thereof and couples said aerial to said first input via a signal path within said coupler from said second port to a third port thereof, the transmitter section output also being coupled to said second input.
Continuous-wave radar systems are well-known and discussed in many text-books. A continuous RF signal from a transmitter section is radiated from an aerial and, if it should encounter a reflecting target, the reflected signal received by the system is mixed in a receiver section with a sample of the currently transmitted signal. The result of the mixing process contains information about the range and/or the velocity of the target. Thus, for example, if the radiated signal has a constant frequency the reflected signal may have a different frequency to the radiated signal due to the Doppler effect, so that the result of the mixing process is a beat frequency signal having a frequency which is representative of the relative velocity of the radar system and the target. As another example the reflected signal received at any given time may have a different frequency to that currently being radiated, due to frequency sweeps being imparted to the radiated signal and the finite time delay occurring between radiation of the signal and reception of the reflected same signal. In this case the result of the mixing process is again a beat frequency signal the frequency of which is now, for zero relative velocity between the radar system and the target, representative of the range of the target.
Some of the known systems employ separate aerials for transmission and reception, this assisting in the avoidance of excessive breakthrough of the transmitted signal into the receiver section (where it would be liable to give rise to excessive noise and even damage). However from the point of view of economy and/or compactness it is desirable to use a common aerial for both transmission and reception, and this is done in other known systems, further measures then being taken to avoid excessive breakthrough of the transmitted signal into the receiver section. The general set-up in the latter case is shown in FIG. 1 of the accompanying diagrammatic drawings. In FIG. 1 a (known) continuous-wave radar system has a transmitter section 1, a receiver section 2, and a transmit/receive aerial 3 which is common to both of the sections 1 and 2. The receiver section 2 includes a mixer arrangement 4 for mixing a signal derived from the aerial 3 and applied to a first input 5 thereof with a signal derived from the transmitter section 1 and applied to a second input 6 thereof. The output 7 of the mixer 4 is fed to a signal processing arrangement 8 which also forms part of the receiver section 2. The manner in which the processing arrangement 8 operates is irrelevant in the present context and will therefore not be discussed. An output 9 of the transmitter section 1 (which section basically comprises a continuous-wave r.f. signal generator) is coupled to the aerial 3 via a first port 10 and a second port 11 of a device 12 and a signal path 15 within the device 12 from the port 10 to the port 11. Moreover the aerial 3 is coupled to the input 5 of mixer 4 via the port 11 and a third port 13 of the device 12 and a signal path 16 within the device 12 from the port 11 to the port 13. The output 9 of the transmitter section 1 is also coupled to the second input 6 of mixer 4 via a directional coupler 14, so that a portion of the output signal of transmitter section 1 is fed to the mixer input 6.
As discussed, for example, on pages 71-72 of the book "Introduction to Radar Systems" by M. Skolnik (2nd edition), device 12 serves to provide some degree of isolation of the input 5 of mixer 4 from the output signal of the transmitter section 1. To this end it may be formed by, for example, a hybrid junction or a circulator. A circulator has the advantage that it can be made to direct substantially the whole of the signal power applied to its port 10 from the transmitter section output 9 to its port 11 and thence to the aerial 3, and to direct substantially the whole of the signal power applied to its port 11 from the aerial 3 to its port 13 and thence to the mixer input 5, with the result that minimal signal power need be wasted. However circulators are difficult to set up and difficult to integrate with other microwave components. On the other hand a hybrid junction (which is a four terminal-pair device which ideally has the property that power supplied to a given terminal is divided, usually equally, between two of the three remaining terminal pairs and nothing is coupled to the fourth terminal-pair and which may be constituted, for example, by a 3 dB directional coupler with a symmetrical coupling element) while being easier to set up and integrate, inherently results in a waste of power if used as the device 12 of FIG. 1. Energy from the transmitter section 1 will divide equally between the aerial 3 and the matched termination on the otherwise unused arm of the device 12, whereas energy received by the aerial 3 will divide equally between the mixer input 5 and the transmitter section 1. Thus there will be a minimum loss of 6 dB of signal power (3 dB on transmission and 3 dB on reception) as mentioned in the part of the Skolnik book referred to above and also, for example, on page 890 of the book "Microwave Engineering" by A. F. Harvey (1963). It is an object of the invention to enable this minimum loss to be reduced without it being necessary to employ a circulator.